Phenol is commonly manufactured from cumene, wherein cumene is oxidized to cumene hydroperoxide (CHP) and the resulting oxidation product mixture is concentrated and subjected to a cleavage reaction. Subsequently, the cleavage product mixture is conducted to a distillation section, wherein the main products of the cleavage reaction, i.e. phenol and acetone, are first separated and then purified through a series of distillation or other purification steps. This is the cumene process.
The cleavage of cumene hydroperoxide to phenol and acetone is often catalysed using an acid meaning that the resulting cleavage product mixture contains not only acid catalyst residues but organic acid salts which interfere with the subsequent separation and purification of phenol and acetone. The organic acids may include, for example, formic acid and benzoic acid. Before the distillation of the main products of the cleavage reaction (i.e. phenol and acetone), these acids have to be neutralized to prevent, for example, side reactions and/or equipment corrosion during the distillation process.
Neutralization may be accomplished by treating the acids with a basic material, such as an aqueous alkaline agent, typically NaOH (caustic). During the neutralization reaction, salts are formed. However, these too can cause problems during the subsequent purification and recovery of phenol and acetone by distillation. For example, salt carryover to the distillation section causes fouling and down time, and thus capacity reduction.
The neutralization of the cleavage product mixture produced in a phenol production process has generally been conducted by adding a caustic agent to the mixture or by adding an aqueous salt solution optionally containing excess caustic agent, as in U.S. Pat. No. 6,066,767. This publication concerns a method for purifying CHP cleavage products in a purification process system. Purification is performed by extracting hydroxy acetone and carbonyl compounds with a water-salt solution at the neutralization stage by adding a caustic agent.
U.S. Pat. No. 3,931,339 concerns a process for the removal and neutralization of an acid catalyst and organic acid by-products from the reaction mixture resulting from cumene hydroperoxide cleavage. The process comprises extracting the acids from the organic phase into an aqueous phase and neutralizing them using an aqueous solution of an acidic or neutral inorganic salt, and an excess of alkali metal hydroxide or alkali metal phenate, separating the aqueous phase containing the neutralized acids from the mixture, then contacting the organic phase with an aqueous solution of an acidic or neutral inorganic salt and a weak acid, and thereafter again separating the aqueous phase from the mixture.
GB 743,004 concerns a process for the production of phenol and acetone from CHP, in which the acid and acidic organic impurities in the cleavage mixture are neutralized using an alkaline material. Corresponding salts of the acid and acidic impurities are then partly precipitated in the effluent, whereby they are removed by adding water to the neutralized cleavage mixture in an amount sufficient to cause formation of two phases. The aqueous phase will then contain substantially all of the salts. The salt content of the organic phase will, according to the publication, be less than 0.03% by weight, often as low as about 0.01% by weight.
Neutralisation of the cleavage product is thus usually carried out using fresh caustic solution and that can lead to high levels of caustic use. Also, when only fresh caustic is used in cleavage product neutralisation, the consequences are following:                1) The amount of wastewater from the plant is increased as so much is used to dilute the caustic down to appropriate concentration for neutralisation;        2) Sulphuric acid consumption is increased as the wastewater has to be neutralised (with sulphuric acid) before the de-phenolation step;        3) The steam consumption in the de-phenolation is increased (the more wastewater the more steam is required for de-phenolation step).        
The neutralisation of the cleavage product from acid catalysed cleavage is not however, the only use of caustic in the phenol product process. In U.S. Pat. No. 5,220,103 and U.S. Pat. No. 5,120,902, caustic is used to wash recycled cumene. In WO2009/080341 and WO2010/069586, caustic is used to wash oxygen gas used for the oxidation reaction.
Caustic is also used, however, as a washing solution during hydrocarbon recovery, in particular during the recovery of the bottoms fraction from acetone distillation.
Once the cumene hydroperoxide has been cleaved to form phenol and acetone and once the reaction products have been neutralised, the phenol and acetone are separated by distillation and sent to separate parts of the plant for further purification. The acetone fraction, containing various impurities, is sent to a column for further purification as the acetone at this stage in the process is still intimately mixed with impurities such as water, unreacted cumene, organic acid salts, some sodium hydroxide and some residual phenol. These impurities must be removed from the acetone before it can be commercialised. Also, phenol and cumene are valued resources and are preferably recycled/isolated from the acetone fraction.
The present inventors have made a series of innovations to improve the production process here. Firstly, it has been surprisingly found that the hydrocarbon wash step (i.e. the washing of the bottoms from the acetone distillation column) is improved if two different concentrations of caustic are used in separate steps. In particularly a lower then higher concentration caustic solution should be employed. It is further noted that improvements can then be achieved if the twice washed hydrocarbons are washed once more with water. For process efficiency the water used in the washing step can be extracted from the washing step and used to dilute the caustic which is used for the second washing step. Caustic is typically supplied industrially at 50 wt % strength which is too high (and too expensive) for use industrially.
Moreover, the caustic used in the second washing step can be further diluted with water to act as the caustic for the first washing step. That water can again be derived from the third washing step or may simply be present in the bottoms fraction inherently.
As a final advantage, the concentration of caustic used in the first washing step is also at an ideal level for use in the neutralisation step which takes place after the cumene hydroperoxide cleavage reaction. The use therefore of a recycle of the first caustic wash from acetone bottoms washing to the neutralisation step forms a still yet further aspect of the invention. It has been surprisingly found, that instead of using only fresh caustic for that neutralisation, caustic from the hydrocarbons wash can be used to replace part or all of the fresh caustic typically used. That has major advantages in terms of raw material costs.
The present inventors have therefore devised an improved process which maximises the use of raw materials such as water and caustic, which on the scale used in industrial plants offers major economic benefits.